As a switching power supply circuit capable of high speed operation, a proposal has been made for the one configured to adjust an output voltage by turning on or off a switching element with the use of the output of a flip-flop circuit, as shown in FIG. 3. As shown in this drawing, this switching power supply circuit is equipped with a comparator 1 which compares a feedback voltage FB based on an output voltage VOUT (i.e., a voltage obtained by dividing the output voltage VOUT by a resistance ratio between the resistances of feedback resistors RFB1 and RFB2) with a reference voltage Vref as the output voltage of a reference power supply V0; a flip-flop circuit 2 which is set by the comparator 1; and an ON-time control circuit 3 which resets the flip-flop circuit 2 at a time when a predetermined time elapses after an output signal from the flip-flop circuit 2 has fallen. The output signal of the flip-flop circuit 2 is supplied to a main switching element (in the present example, a P-channel MOSFET) SW1 and a subordinate switching element (in the present example, an N-channel MOSFET) SW2 via a buffer circuit 4, whereby the main switching element SW1 and the subordinate switching element SW2 connected in series via a connection point Lx are turned on or off by a synchronous rectification method. An input voltage VIN is applied to the input side of the main switching element SW1.
In this manner, the output voltage VOUT is obtained which is a predetermined direct current voltage smoothed by a capacitor CL via an inductance coil L connected to the connection point Lx.
In the above-mentioned switching power supply circuit, the feedback voltage FB and the reference voltage Vref are compared in the comparator 1, and the setting of the flip-flop circuit 2 is made by the output of the comparator 1, whereby the on-off control of the switching element SW is performed. If the ripple component of the output voltage VOUT is small, therefore, switching control in the flip-flop circuit 2 becomes unstable. That is, when the ripple component of the output voltage VOUT is small, the difference of the feedback voltage FB from the reference voltage Vref is not at a sufficient level, so that the timing for the setting of the flip-flop circuit 2 deviates from a position on a time axis at which the setting should be made. As a result, deviations are also caused to the timings of the rise and fall of a pulse signal to be supplied from an output terminal Q_B to the main switching element SW1 via the buffer circuit 4 in synchronization with the above proper timing, and to the timing of the rise of an ON-time signal to be delivered via the ON-time control circuit 3. Consequently, the output voltage VOUT shows an unstable action, such as including undulations. Such phenomena become more marked, when a capacitor with a low ESR (equivalent series resistance) is used as the capacitor CL, or when a switching frequency is raised.
In the switching power supply circuit shown in FIG. 3, an integration circuit composed of a resistor R1 and a capacitor C1 is connected in parallel with the inductance coil L to generate a voltage Vc at a junction between the resistor R1 and the capacitor C1, and this voltage Vc is presented as feedback to the input side of the comparator 1 via a capacitor CFB. The voltage Vc has a waveform similar to a current flowing through the inductance coil L. Thus, a state equivalent to a state where the ripple component of the output voltage VOUT is sufficiently great is formed on the input side of the comparator 1. Hence, the aforementioned unstable action of the output voltage VOUT can be avoided, and the stabilization of switching operation in the flip-flop circuit 2 can be achieved, even when the ripple component of the output voltage VOUT is small.
In the switching power supply circuit shown in FIG. 3, however, the responsiveness of the circuit varies according to the amplitude of the ripple component of the output voltage VOUT. Thus, the amplitude is adjusted to be about several tens of millivolts. As a result, if the amplitude is great, a transient response will deteriorate. If the amplitude is small, by contrast, an unstable action will occur.
In order to operate this type of switching power supply circuit at a high oscillation frequency of several MHz, the speed of the comparator 1 needs to be enhanced. Thus, the problem arises that current consumption increases significantly, and because of a small DC gain, load stability lowers. Moreover, a phase shift occurs owing to a response delay of the comparator 1, and stable operation may fail to be performed.
Under these circumstances, proposals have been made, as disclosed in JP-A-2011-176990 and JP-A-2011-147324, for a switching power supply circuit which varies the reference voltage (JP-A-2011-176990), and a switching power supply circuit which detects a current flowing through a switching element in an attempt to stabilize operation (JP-A-2011-147324). Both of these switching power supply circuits, however, pose the problem that load stability deteriorates.
JP-A-2010-252627 is available for teaching a conventional technology which can speedup the operation and can improve load stability as well. In incorporating a resistor and a capacitor of a CR integration circuit into an IC as in JP-A-2010-252627, however, there is need to set the values of the resistor and the capacitor at those values which make the influence of the parasitic capacitance of the resistor negligible. However, this requires a very large layout area.
The present invention has been accomplished in the light of the above-mentioned earlier technologies. It is an object of the invention to provide a switching power supply circuit of a PFM control type which can utilize a capacitor with low ESR, such as a ceramic capacitor, for load capacity, is capable of stable operation even at an oscillation frequency of several MHz or more, obtains high load stability, and enables a layout area to be reduced.